[1]}, where the peak frequency is measured in GHz. These are found to be \(B_{\rm SSA}^A = 0.9\)  G and \(B_{\rm SSA}^D = 2.6\)  G for hotspots A and D, respectively. As has been previously discussed, these magnetic field strengths are \(\sim 10^4\)  times higher than has been estimated from minimum energy arguments (i.e. \(\sim 300\)  \(\mu \) G; [2]}) or synchrotron-self Compton modelling (i.e. \(\sim 200\)  \(\mu \) G; [3]}). The synchrotron-self absorption model (green dot-dashed line in Fig. REF ) does not fit the data at 2 GHz, where the observed spectrum steepens, because we do not incorporate a cooling break in the model. This is because with magnetic field strengths of order 1 G, the radiative cooling timescale is prohibitively small, and cannot be the cause of the break in the spectrum observed at GHz frequencies; the break frequency \(\nu _b \sim 10\)  GHz implies a cooling time in the order of months, which is physically impossible given the 2 kpc diameter of the hotspots.
